Protection under the rain

Has anyone a suggestion for a protective bag or case that would allow the use of a digital camera under moderate rain ?
I have heard about a shower cap ! Not that I want something fancy, but there must be a more efficient way (how do you turn knobs through a shower cap ?).

You can use a plastic bag. Cut a hole in the bottom and use elastic bands to hold it on the lens. A lens hood would be a good idea to prevent water from hitting the glass. You can put your hands inside the bag and press the buttons.
Some people use the leg from a pair of water proof trousers.
You can buy proper covers for wet weather, but I'd rather use the bag.